12.10.1 What Unreading Means

Here is a pictorial explanation of unreading. Suppose you have a
stream reading a file that contains just six characters, the letters
‘foobar’. Suppose you have read three characters so far. The
situation looks like this:

f o o b a r
^

so the next input character will be ‘b’.

If instead of reading ‘b’ you unread the letter ‘o’, you get a
situation like this: